The present invention relates to a rotary developing apparatus having a plurality of developing devices mounted along the outer periphery of a rotary unit with a cylindrical shape.
In conventional full-color image forming apparatus adopting the rotary developing method, a plurality of developing devices are mounted along the outer periphery of a rotary unit, and each developing device is successively revolved to a developing position to perform a developing operation. For this purpose, driving means for rotating the rotary unit and driving means for rotating a developing roller contained in each developing device on the rotary unit are provided separately from each other.
The above-described rotary unit equipped with a plurality of developing devices is generally in the shape of an approximately circular cylinder and has heavy members mounted near the outer periphery of the circular cylinder, such as developing rollers serving as developer carriers, which are metallic rollers or rollers each comprising a metallic core and an elastic material covering the core, to develop a latent image formed on a latent image carrier, e.g. a photosensitive member. Therefore, the rotary unit has a large moment of inertia.
In a general full-color printing operation using developing devices for four colors, for example, 90-degree rotation of the rotary unit is performed four times, whereby the developing devices for four colors are successively moved to a developing position at which each developing device faces the photosensitive member to perform a developing operation. After being stopped at the developing position for performing a developing operation, the rotary unit is held in this position, for example, by using the holding force of the motor, or an engagement member provided separately.
In the 90-degree rotating operation, when the inertia moment of the rotary unit is large, the motor used as a drive source needs to generate correspondingly large force. Further, an effective way of increasing the printing speed of the apparatus is to increase the speed of the 90-degree rotating operation. However, if the speed of the 90-degree rotating operation is increased, acceleration acting during the rotation increases correspondingly. Consequently, force required from the drive source becomes greater because the force for rotation acts against the moment of inertia with the square of acceleration.
The force for rotation exerts an influence adversely when the rotation of the rotary unit is stopped. To stop the rotation of the rotary unit, the drive source functions as a brake to damp the rotational force of the rotary unit. Ideally, it is desirable that the rotational force should be made zero by the braking force immediately before the rotary unit comes to a stop.
In actual practice, however, the rotational force undesirably remains owing to backlash and play in the gear train of the driving system, deflection, torsion of the rotary unit, etc. The residual rotational force is transmitted to the whole apparatus as vibration through the drive source when the rotary unit is stopped. The vibration is transmitted to the exposure means and the latent image carrier, causing displacement to occur during the formation of a latent image. If the vibration is transmitted to the transfer part, transfer displacement may occur.
When the driving means for rotating the developing roller starts its operation immediately after the rotation of the rotary unit has stopped, unevenness of rotation of the driving means or vibration occurring at the drive source is transmitted to the whole apparatus through the drive source. The vibration causes image defects to occur owing to displacement, etc. as in the case of the vibration generated at the time of stopping the rotary unit.
The above-described two drive sources have respectively different rotation and vibration characteristics when the drive sources are different in type or lot from each other. Even if the two drive sources have substantially equal vibration characteristics, if they are installed at different places, there will be influence of vibration characteristics of the places where the drive sources are installed. Therefore, vibrations from the two drive sources are likely to consist of different components and hence remain without damping. Under certain circumstances, the vibrations are combined together in such a manner as to be superimposed on one another and thus amplified. This causes image defects such as displacement over a long period.